1. Field
The present invention includes methods, apparatus and systems related to the bonding area design for transient liquid phase (TLP) bonding processes. For example, the present invention improves bonding quality and fabrication reliability of bonding technologies for electronic devices.
2. Description of the Related Art
TLP bonding is a known process for bonding together various materials and devices. Such a process has been used in the manufacturing of electronic devices incorporating power electronics, MEMS sensors, SiC devices, SAW sensors, LEDs, and the like. For example, TLP may be used to bond a wafer to another wafer (e.g., wafer-to-wafer bonding) or to bond a die to a substrate (e.g., die-to-substrate bonding). FIG. 1A-FIG. 1D illustrate a traditional four-step process for TLP bonding. Initially, as shown in FIG. 1A, a first material 105 may be bonded to a second material 115 (which may be the same or different material as the first material 105) via a third material 110. FIG. 1B illustrates what happens when heat is applied to the third material 110. In practice, the third material 110 has a melting point that is substantially lower than the first material 105 and the second material 115, and as such, the third material 110 will melt before the first material 105 and the second material 115 melt. As the third material 110 melts, it diffuses into the first material 105 and the second material 115 such that layers 120 and 125 may become a physical blend of the materials 105 and 110, and materials 115 and 110, respectively, whereas layers 130 and 135 may remain predominantly the third material 110. As the diffused materials continue to sequentially react, FIG. 1C illustrates resulting alloys 140 and 145 which are formed via isothermal solidification. The solidification continues until the bondline becomes homogenous as a mixture of materials 105, 110 and 115 shown as bondline alloy 150 in FIG. 1D. In this manner, the third material 110 bonds together the first material 105 and the second material 115. Although not shown, large mechanical pressure or air pressure control may also be applied during the TLP bonding process.
Because the bondline alloy 150 provides excellent electrical and thermal conductivity, among other advantages, much effort has been focused on uniform bonding and improving the bondline. As such, there has been little advancement made to address the problematic expansion of melted material beyond the bonding area.
This pervading and unavoidable problem with standard TLP bonding is illustrated by FIG. 2 and FIG. 3 where the melted materials (or interlayer) have expanded or moved out of the bonding area due to poor or non-existent expansion limiters. As shown in FIG. 2, the melted materials expand beyond the limits of the entire boundary on all four sides. While not shown, the melted material exceeding the boundary might also be thick enough to unintentionally contact another device further causing problems. Even where the melted material does not exceed the boundary on all four sides as shown in FIG. 3, or become thick enough in the overflow area, unfortunately, such an occurrence may still cause significant damage to the devices to be bonded as well as other production problems. Generally, the flowing or reflowing of the metal to unintended portions of an electrical device may cause an electrical short circuit. Furthermore, because the flowing of the metal is often unpredictable and uncontrolled, an inconsistent or non-uniform product may result. Additional bonding material (e.g., the third material 110) may also be needed to account for losses due to the undesired migration of the metal beyond the bonding area. Furthermore, the problem does not necessarily end at the time of production, but may occur or re-occur if, at the time the device is utilized, heat is produced that again melts the reflowed material.
Accordingly, what is needed is a flow or reflow control to mitigate the unintended movement of the bonding material.